The present invention relates to methods of driving an ink jet recording element of the kind comprising a compression chamber for containing the ink, and in which the volume of the compression chamber is altered by means of a piezo-electric transducer to which a control voltage can be applied and wherein the compression chamber will expand if a control voltage is applied in the direction of the polarisation voltage of the transducer and will contract if a control voltage is applied in the opposite direction. The invention also extends to a circuit arrangement for carrying out the method according to the invention.
The ink jet recording element is used, for example, in printers, teletype machines, facsimile machines and similar devices.
Such an ink jet recording element consists basically of a tubular compression chamber the end of which that is directed towards a recording medium is constructed as writing nozzle and the opposite end of which is joined to a storage container for the ink. The compression chamberis enclosed by a piezo-electric transducer which alters the volume of the compression chamber in synchronism with a control voltage, utilizing the piezo-electric effect.
During a writing phase the compression chamber contracts and the pressure wave arising in this manner propels one of several droplets of ink out of the writing nozzle.
During a subsequent regeneration phase the compression chamber expands and replenishes the amount of ink, reduced due to the formation of the droplets, from the storage container.
The requirement is for a short writing time and a good-quality reproduction. These requirements can be met by a high and constant speed of emission of the droplets of ink from the ink nozzle, by a short-interval and constant sequence of droplets and by a constant droplet volume.
From German Offenlegungsschrift No. 21 44 892 different types of control voltage generators for a piezo-electric transducer are known already in which the control voltage pulses are generated by charging and discharging the piezo-electric transducer, which acts electrically as a "capacitor" via resistors and inductors.
In these control voltage generators the minimum achievable pulse duration is limited with RC circuits by their time constants or with LC circuits by their resonance behaviour. The frequency of the droplet sequence cannot, therefore, be increased at will and the limit of working speed has been reached.
The step rise and exponential decay of the control pulses causes the transitions from volume constriction to volume expansion to be not continuous, leading to the recording element behaving in an uncontrollable manner.
In practice it has also become apparent that drives of these types have a tendency to produce spurious oscillations in the liquid. A mutual interaction of the ink droplets takes place which produces, according to the phase relationship of the spurious oscillations at the time when droplets are being formed, shorter or longer intervals between droplets. In order to keep the effects of such spurious oscillations in the liquid to a low level, certain settling periods for the spurious oscillations have to be included into the calculations for the customary control voltage generators which also sets limits to an increase in the frequency of the droplet sequence.
In addition, the known types of drive do not work stably, producing voltage pulses containing different amounts of energy, and thus droplets of different volume. If the voltage pulses are too small no droplets at all will be emitted from the writing nozzle and if the voltage pulses are too big, on the other hand, there can be spurious oscillations and so-called satellite droplets.
However, a non-uniform droplet sequence and varying droplet volumes have a considerable influence on the quality of the reproduction.
A further disadvantage consists in that, depending on the steepness of the voltage pulses, a high dynamic pulse power must be transmitted which is retained statically after switching has occurred or which decays exponentially.
In German Offenlegungsschrift No. 25 48 691 another drive system for a piezo-electric transducer is described in which the control voltage pulses are applied to the transducer by means of a pulse transformer.
In this case, too, the attainable pulse duration and thus also the attainable droplet repetition rate is limited by the behaviour of the resonant circuit formed from the inductance of the pulse transformer and the capacity of the transducer.
In German Offenlegungsschrift No. 24 05 584 it is suggested to prevent uncontrollable spurious oscillations by means of a special construction of the compression chamber and of the throttling device and by using suitable materials, so that the droplet frequency can be increased. These measures, however, are very expensive. It had not been recognized here that the existing problems have to be solved by driving the piezo-electric transducer in a suitable manner.
It is an object of the invention to lay down a method and a circuit arrangement for driving an ink jet recording element comprising a compression chamber for the ink, operated by a piezo-electric transducer, by means of which the said disadvantages are eliminated and a high and constant droplet emission speed and droplet frequency and a constant droplet volume are achieved. These measures permit the working speed and quality of reproduction of ink jet recording devices to be improved considerably.